The present invention relates to a guide vane which is adapted to be mounted in an annular fluid conveying duct of a gas turbine engine.
A typical gas turbine engine contains several annular fluid-conveying ducts which form e.g. the compressor or turbine sections of the engine.
Each of these ducts contains a number of blades or vanes (which we henceforth call guide vanes) which are circumferantially distributed in one or more guide vane assemblies in the duct. The guide vanes are classed as rotor blades or stator vanes depending on whether or not the respective guide vane assembly rotates in the duct when the engine operates. Each guide vane has an aerofoil part which extends radially across the duct, the aerofoil part having a pressure side and a suction side.
It is usual for the aerofoil part of each guide vane to span radially across the duct in a generally straight line, although a degree of xe2x80x9cleanxe2x80x9d, relative to the straight line extending from the axis of the duct and passing through the radially inward end of the guide vane, is typically introduced to reduce or eliminate corner stalls and suppress or delay secondary-flow formations. GB 712,589 describes an assembly having guide vanes which present C-shaped or cranked profiles (suction side in) when viewed axially along the duct. This type of profile is intended to improve the fluid flow distribution in the annular duct.
An object of the present invention is further to improve the fluid flow in an annular fluid-conveying duct of a gas turbine engine, and thence improve the efficiency of the engine.
In a first aspect the present invention provides a guide vane which is adapted to be mounted in an annular fluid-conveying duct of a gas turbine engine with an aerofoil part of the guide vane extending radially across the duct, wherein, on a transverse cross-section relative to the direction of intended fluid flow across the aerofoil part, at least a portion of the aerofoil part is S-shaped.
We have found that by adopting such an S-shape the pressure loads on the vane are altered relative to conventional vanes, and the amount of secondary flow can be substantially reduced. By secondary flow we mean fluid movement away from the primary flow direction. Secondary flow is generally undesirable as it tends to promote non-uniform fluid movement towards the exit of the vane and enhances mixing loosen.
We believe the S-shape may encourage a vortex having the opposite sense to the duct vortex, which reduce the amount of secondary flow. In any event, modelling results suggest that the S-shape of the present invention and the C-shape of GB 712,559 generate different vane loadings, particularly at low- to mid-span on the aerofoil part of the vane (low-span being closer to the axis of the duct). Advantageously, compared with the C-shape, the S-shape appears to reduce the mid-span loading and hence to reduce the likelihood of flow separation.
The kinetic energy associated with secondary flow is termed the secondary kinetic energy (SKE). Modelling results suggest that the SKE of a highly-loaded, low aspect ratio turbine nozzle guide vane can be reduced by as much as 60% by adopting an S-shape. This can lead to a 2% improvement in vane efficiency. Similar improvements are expected for guide vanes at other engine locations and with higher aspect ratios.
In preferred embodiments, the S-shape is smoothly curved throughout its length, as abrupt changes in geometry can lead to boundary layer thickening and increases in loss. However, in other less preferred embodiments the S-shape may comprise two or more mutually inclined rectilinear subsections. Thus the S-shape may have two sharp corners (i.e. be xe2x80x9cZ-shapedxe2x80x9d) or have a smooth curve and a sharp corner.
The S-shaped portion effectively produces two major changes of direction (i.e. in geometric terms, a maximum and a minimum) in the aerofoil part. Preferably, the guide vane is shaped so that, when the guide vane is mounted in the duct, the major changes of direction are to respective radial sides of the radially mid span position of the duct. This appears to provide the best reductions in SKE and mid-span loading.
However, the acrofoil part may have more than two major changes of direction. For example, a W- or M-shaped portion (i.e. an S-shaped portion having at one end an additional bend or corner) would have three major changes of direction. Thus the S-shaped portion may be sinusoidal with two, three or more maxima and minima.
Preferably, the guide vane is shaped so that, when the guide vane is mounted in said duct and is viewed from the pressure side of the aerofoil part, the radially inner part of the S-shaped portion appears concave and the radially outer part of the S-shaped portion appears convex.
In one embodiment, the leading edge of the aerofoil part has an S-shaped portion, and/or the transverse cross-section midway between the leading edge and the trailing edge of the vane has an S-shaped portion.
In a preferred embodiment, however, the leading and trailing edges of the aerofoil part and all transverse cross-sections therebetween have S-shaped portions.
In a further aspect, the present invention provides a gas turbine engine, or a component of a gas turbine engine, comprising a plurality of guide vanes according to the previous aspect. The component may be e.g. a blisk (a bladed disk), an assembly of circularly arranged guide vanes, or part of such an assembly.